Sunday, November 24, 2013

Performance considerations for the M16 Family

In our last post about the M16 family, there was an interesting comment by a long time reader of this blog, Mr. Wojciech Imbierowicz. He questioned the ballistic performance of bullets on the M4 rifle, as compared to the M16A2. That will be the subject of today's post.

The first thing we will discuss is the relationship of bullet velocity to barrel length. When a cartridge is fired, the pressure of the expanding gases in the barrel is responsible for accelerating the bullet. As soon as the bullet leaves the front end of the barrel, the gases stop acting upon it. Therefore, the maximum velocity of the bullet is at the tip of the barrel and it starts to decelerate after it leaves the barrel. Obviously, if we have a shorter barrel, then the gases will not have a chance to accelerate the bullet much, before it exits the barrel. If we have a longer barrel, then the expanding gases will act on the bullet for a longer time and be able to accelerate it to higher velocities. Of course, there is a certain point in the barrel length, beyond which, increasing the length produces diminishing gains in the velocity of the bullet. Also, if a barrel is too short, the propellant may not entirely burn inside the barrel, which also reduces the force that is acting upon the bullet. On a longer barrel, the entire propellant may burn inside the barrel.

Depending on the type of cartridge, the optimum barrel length for maximum velocity may vary. The following table shows us some numbers for two common cartridges used by the M16 family, the M193 (the original cartridge that was used with the M16A1) where the bullet weighs 55 grains, and the SS109 (the NATO standard cartridge), where the bullet weighs 62 grains.

Barrel Length
(in inches)

Velocity (M193)
(Feet per second)

Velocity (SS109)
(Feet per second)

10

2739

2627

11.5

2872

2738

14.5

3064

2907

16

3132

2989

20

3259

3095

24

3315

3158

26

3391

3231

As we can see from the above chart, barrels that are between 16 and 20 inches in length seems to be the sweet spot, after which, the increase in barrel length doesn't increase the velocity of the bullet by that much.

Now on to one more feature of the bullets used by the M16 family. The 5.56x45 mm. bullet is pretty small, but it has an interesting property that if it is travelling above a certain velocity when it hits a target, it tends to fragment and produce a wound much larger than the size of the bullet. Below this velocity, the bullet does not fragment reliably and produces a much smaller wound. For the M193 cartridge, this threshold is approximately 2700 feet/sec and for the SS109, it is approximately 2500 feet/sec. Of course, the material of the bullet and its construction also have a lot to do with how much velocity they need to fragment reliably. For the purposes of this discussion, we will only consider the two above-mentioned cartridges.

Now, from the table above, we see that if the barrel is 10 inches long, the velocity of the M193 bullet (2739 feet/sec) is just barely above its the fragmentation velocity (2700 feet/sec). The same is true for the SS109 bullet as well. Therefore, for both these cartridges, the bullet's velocity will fall below the fragmentation velocity pretty quickly beyond about 20 meters or so. For the record, the 10 inch barrel is used by some civilian and police forces AR-15 models.

With the 14.5 inch barrels (such as those used by the military's M4 carbine), we see that the velocities are somewhat higher (3064 feet/sec for M193 and 2907 feet/sec for SS109). This means that the bullets will travel about 100 meters before their velocities fall below the fragmentation threshold velocity.

Now, with the 20 inch barrels (such as those commonly used by AR-15s, M16A2 etc.), the velocities are a bit higher and therefore, the bullets can travel about 150-200 meters before they will stop fragmenting reliably. Therefore, between 100 to 200 meter distances, they generally have better wounding performance than bullets fired out of a M4 carbine, which only has a 14.5 inch long barrel.

To improve the performance, the US military has come up with some newer bullet designs that have lower fragmentation velocity thresholds. The newer M855A1 and the Mk 318 cartridges were specifically designed to handle some of the shortcomings of the previous cartridges.